1. Statement of the Technical Field
The present invention relates to merchandise surveillance systems and more particularly to a system and method for monitoring the position of a door-mounted antenna, and altering the interrogation zone created by the antenna, depending upon the angle which the door containing the antenna is positioned relative to a reference plane.
2. Description of the Related Art
In a surveillance system, antennas such as EAS (“Electronic Article Surveillance”) antennas or RF (Radio Frequency) antennas, transmit interrogation signals that are received by markers such as Radio Frequency ID (RFID) or magneto acoustic markers located on merchandise within an establishment. The markers send corresponding signals back to the antenna. Thus, the interaction between the antennas and the markers establish an interrogation zone that can provide an establishment, such as a retail store, with a security system for its merchandise. Conventional surveillance systems include antennas located in a pedestal, the floor, the ceiling or wall or a combination of each such that the antennas can be used to monitor a large volume with the minimum number of antennas. While these types of systems are fine for large department stores and supermarkets, small shop retailers have different concerns since their security budgets may be lower and floor space may be at a great premium.
One solution to the aforementioned problem faced by small retail stores is to mount one or more antennas on a swinging or sliding door. This allows retailers to utilize valuable floor, wall and/or counter space of merchandise, while still maintaining a security system. However, a problem that arises with this solution is that when the door is opened, the door-mounted antenna moves, and the resulting detection zone that is generated by the antenna also moves, possibly resulting in areas that now become out of reach of the antenna's detection zone. This is not a desired result in a small store that needs merchandise as close to the door exists as possible.
A problem that arises when antennas are mounted on moving doors is that as the door is opening or closing, the antenna also moves, thus altering the originally-designed interrogation zone. The resulting location of the antenna may result in an over-range or an under-range condition. An over-range condition occurs when the magnetic field from the antenna covers a range of areas that includes locations where detection coverage is not needed. For example, a customer should be allowed to wait on line and approach a register holding an item having a marker without an alarm being set because the marked item has moved within the interrogation zone. This might occur when a door is swung into the store by the entrance of a new customer, and/or the exit of an existing customer, and the range of the magnetic field radiating from a door-mounted EAS antenna, or the range of RF signals transmitted by an RF antenna, which is moving along with the swinging door, coincides with the signal transmitted by the marker on an item being purchased by another customer on a check-out line. An over-range situation may also occur when the line for the cash register passes by an interrogation zone, or when a sliding door with an antenna moves laterally, moving marked items into the interrogation zone.
An under-range condition occurs, for example, when a customer is exiting the store by swinging the door outward. In this case, because the antenna has moved along with the door, its interrogation zone may not cover a marked item near the exit of the store. Thus, a shoplifter who is approaching the door with a marked item when the door is opened by a new customer or an existing customer exiting the store, will not trigger an alarm since the door upon which the alarm is mounted has been swung outward, and the unadjusted magnetic field no longer coincides within signal transmitted by the marked item.
Therefore, what is needed is a method and system that determines when a door containing an antenna is opened, in which the angle that the door is opened is monitored and measured with respect to a given reference plane, and controls the interrogation zone of the antenna to account for over-range and under-range conditions.